A cellular telephone is a very convenient business accessory which permits use of a radio operated telephone in a motor vehicle. The cellular telephone is equipped with an annunciator circuit coupled to a small loudspeaker or equivalent, which circuit is triggered by a "ringing signal" in the radio signal received by the telephone. The small loudspeaker or equivalent generates an adequate sound signal when the vehicle is being driven, or the vehicle driver/occupant is close to the car in a quiet area. However, if the driver/occupant is some distance from the car, that is the car is being left unattended, sometimes in a noisy area, the normal ringing sound or signal from the cellular telephone will be inaudible and the incoming call will be lost. The problem of how to signal the occurrence of an incoming telephone call when the vehicle is left unattended is usually solved by an installer. Invariably, the installer will connect the vehicle's horn directly to the cellular telephone. Thus, when the vehicle is left unattended and an incoming call is received, the cellular telephone will cause the vehicle's horn to sound. Using the vehicle's horn to signal an incoming call can be disadvantageous because in some vehicles the horn will not operate if the vehicle's ignition circuit is not energized.
Also, for the installer to wire the horn to the cellular telephone, modifications to the vehicle wiring must be made which can void the car's warranty. Car buyers may be reluctant to void their warranty and therefore may not be able to enjoy all the benefits of a cellular telephone.
In addition, car horns generally emit an offensive sound often startling or annoying persons standing nearby. In fact, because of the offensive sound emitted by car horns, some municipalities will not allow the use of horns, sirens or bells to be used to signal an incoming call on a cellular telephone. Persons wishing to use cellular telephones in these areas must have some alternate way of signalling an incoming call when away from their vehicle.